1. Field of the Invention:
This invention relates to an image reader for reading an image recorded on a microfilm. More particularly, this invention relates to an image reader which possesses a function of projecting an image on the microfilm onto a screen by virtue of light from a light source and a function of transforming the image into electric signals with a photoelectric transferring device.
2. Description of the Prior Art:
Such an image reader as disclosed in Japanese Patent Laid-Open SHO No. 62(1987)-5,764, for example, is used for the purpose of projecting onto a screen a prescribed image retrieved from a multiplicity of images recorded on a microfilm and storing the retrieved image in a memory medium such as a laser disc or printing the image. The image reader of this principle is basically provided with a screen for receiving a projected image, a photoelectric transferring element such as a CCD (charge coupled device) for transforming the exposed image into electric signals, and an image sensor or line sensor. This image reader incorporates therein a movable switching mirror capable of alternate selection between a reader optical path for projection of the image on the screen and a transforming optical path for projection of the image on the aforementioned photoelectric transferring element. These two optical paths are formed with one and the same light source.
When the image is to be projected on the screen, the illuminance of the light source must be fixed on the order of 300 to 500 luxes in order that an operator will be allowed to make an effective observation of the image. In contrast, the CCD as a photoelectric transferring element is saturated by exposure to a illuminance of about 30 to 40 luxes. When it is exposed to a illuminance exceeding this level, it is no longer able to transfer an image information into electric signals. The CCD, therefore, must be adapted so as to be exposed to the image with an illuminance not exceeding this level. Particularly when an image containing a photographic information of halftone pattern recorded in a microfilm is to be exposed on the photoelectric transferring element, the luminous energy of the light source must be extremely lowered as compared with that required for the reader mode. To permit the notable alteration of the luminous energy during the reader mode for the projection of the image on the screen and during the transferring element, therefore, the voltage applied to the light source must be changed very largely. This change in the volatage supply has been a major cause for early termination of the service life of the power source.
During the transfer of an image information on a microfilm into electronic signals, the brightness of the light source must be varied in a multiplicity of stages so that the light of the optimum luminous energy conforming with the density of the aforementioned image will be always exposed onto the photoelectric transferring element, with the illuminance kept within a range lower than that required during the projection of the image on the screeen. The light source must be adapted so that even during the projection of the image on the screen, the operator will be allowed to vary the brightness of the light emitted thereby. This control of the brightness of the light source is effected by actuating a switching element with a control signal issued from a microcomputer thereby varying the proportions of the time during which a prescribed voltage is applied by the power source to the light source and the time during which the application of the voltage is absent so as to permit regulation of average magnitude of the voltage supplied to the light source.
In the conventional image reader constructed as described above, while the image reader is in operation, particularly in the aforementioned reader mode, the operator is compelled to feel offensive noise because the movable members of the image reader are kept immobilized. It has been demonstrated that this noise is caused by the ON-OFF frequency of the aforementioned switching element which is operated for the control of the luminous energy or illuminance of the light source. To be specific, the aforementioned ON-OFF frequency or the intermittent frequency is so set that during the transferring mode, the luminous energy will be varied in 500 stages, depending on the density of the image. This frequency happens to fall within the range of sonic frequency. The noise occurs because the control of the luminous energy during the reader mode is carried out in the same range of frequency.